Synchronous TDM-based communication in dominant interference scenarios

ABSTRACT

Techniques for supporting communication in a heterogeneous network are described. In an aspect, communication in a dominant interference scenario may be supported by reserving subframes for a weaker base station observing high interference from a strong interfering base station. In another aspect, interference due to a first reference signal from a first station (e.g., a base station) may be mitigated by canceling the interference at a second station (e.g., a UE) or by selecting different resources for sending a second reference signal by the second station (e.g., another base station) to avoid collision with the first reference signal. In yet another aspect, a relay may transmit in an MBSFN mode in subframes that it listens to a macro base station and in a regular mode in subframes that it transmits to UEs. In yet another aspect, a station may transmit more TDM control symbols than a dominant interferer.

The present application is a Divisional Application of U.S. applicationSer. No. 12/499,423, filed Jul. 8, 2009, entitled SYNCHRONOUS TDM-BASEDCOMMUNICATION IN DOMINANT INTERFERENCE SCENARIOS which claims priorityto provisional U.S. Application Ser. No. 61/080,025, entitled “ENABLINGCOMMUNICATIONS IN THE PRESENCE OF DOMINANT INTERFERER,” filed Jul. 11,2008, assigned to the assignee hereof and incorporated herein byreference.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to techniques for supporting communication in a wirelesscommunication network.

II. Background

Wireless communication networks are widely deployed to provide variouscommunication services such as voice, video, packet data, messaging,broadcast, etc. These wireless networks may be multiple-access networkscapable of supporting multiple users by sharing the available networkresources. Examples of such multiple-access networks include CodeDivision Multiple Access (CDMA) networks, Time Division Multiple Access(TDMA) networks, Frequency Division Multiple Access (FDMA) networks,Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA)networks.

A wireless communication network may include a number of base stationsthat can support communication for a number of user equipments (UEs). AUE may communicate with a base station via the downlink and uplink. Thedownlink (or forward link) refers to the communication link from thebase station to the UE, and the uplink (or reverse link) refers to thecommunication link from the UE to the base station.

A base station may transmit data and control information on the downlinkto a UE and/or may receive data and control information on the uplinkfrom the UE. On the downlink, a transmission from the base station mayobserve interference due to transmissions from neighbor base stations.On the uplink, a transmission from the UE may cause interference totransmissions from other UEs communicating with the neighbor basestations. The interference may degrade performance on both the downlinkand uplink.

SUMMARY

Techniques for supporting communication in a dominant interferencescenario and for supporting operation of a relay station in aheterogeneous network are described herein. The heterogeneous networkmay include base stations of different transmit power levels. In adominant interference scenario, a UE may communicate with a first basestation and may observe high interference from and/or may cause highinterference to a second base station. The first and second basestations may be of different types and/or may have different transmitpower levels.

In an aspect, communication in a dominant interference scenario may besupported by reserving subframes for a weaker base station observinghigh interference from a strong interfering base station. An eNB may beclassified as a “weak” eNB or a “strong” eNB based on the received powerof the eNB at a UE (and not based on the transmit power level of theeNB). A UE can then communicate with the weaker base station in thereserved subframes in the presence of the strong interfering basestation.

In another aspect, interference due to a reference signal in theheterogeneous network may be mitigated. A first station (e.g., a basestation) causing high interference to or observing high interferencefrom a second station (e.g., a UE or another base station) in theheterogeneous network may be identified. In one design, interference dueto a first reference signal from the first station may be mitigated bycanceling the interference at the second station (e.g., the UE). Inanother design, interference to the first reference signal may bemitigated by selecting different resources for sending a secondreference signal by the second station (e.g., another base station) toavoid collision with the first reference signal.

In yet another aspect, a relay station may be operated to achieve goodperformance. The relay station may determine subframes in which itlistens to a macro base station and may transmit in amulticast/broadcast single frequency network (MBSFN) mode in thesesubframes. The relay station may also determine subframes in which ittransmits to UEs and may transmit in a regular mode in these subframes.The relay station may send a reference signal in fewer symbol periods ina subframe in the MBSFN mode than the regular mode. The relay stationmay also send fewer time division multiplexed (TDM) control symbols in asubframe in the MBSFN mode than the regular mode.

In yet another aspect, a first station may transmit more TDM controlsymbols than a dominant interferer in order to improve reception of theTDM control symbols by UEs. The first station (e.g., a pico basestation, a relay station, etc.) may identify a strong interferingstation to the first station. The first station may determine a firstnumber of TDM control symbols being transmitted by the stronginterfering station in a subframe. The first station may transmit asecond (e.g., the maximum) number of TDM control symbols in thesubframe, with the second number of TDM control symbols being more thanthe first number of TDM control symbols.

Various aspects and features of the disclosure are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication network.

FIG. 2 shows an exemplary frame structure.

FIG. 3 shows two exemplary regular subframe formats.

FIG. 4 shows two exemplary MBSFN subframe formats

FIG. 5 shows an exemplary transmission timeline for different basestations.

FIGS. 6 and 7 show a process and an apparatus, respectively, formitigating interference in a wireless communication network.

FIGS. 8 and 9 show a process and an apparatus, respectively, foroperating a relay station.

FIGS. 10 and 11 show a process and an apparatus, respectively, fortransmitting control information in a wireless communication network.

FIG. 12 shows a block diagram of a base station or a relay station and aUE.

DETAILED DESCRIPTION

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA network may implement aradio technology such as Global System for Mobile Communications (GSM).An OFDMA network may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDM®, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). cdma2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the wireless networks andradio technologies mentioned above as well as other wireless networksand radio technologies. For clarity, certain aspects of the techniquesare described below for LTE, and LTE terminology is used in much of thedescription below.

FIG. 1 shows a wireless communication network 100, which may be an LTEnetwork or some other wireless network. Wireless network 100 may includea number of evolved Node Bs (eNBs) 110, 112, 114 and 116 and othernetwork entities. An eNB may be a station that communicates with the UEsand may also be referred to as a base station, a Node B, an accesspoint, etc. Each eNB may provide communication coverage for a particulargeographic area. In 3GPP, the term “cell” can refer to a coverage areaof an eNB and/or an eNB subsystem serving this coverage area, dependingon the context in which the term is used.

An eNB may provide communication coverage for a macro cell, a pico cell,a femto cell, and/or other types of cell. A macro cell may cover arelatively large geographic area (e.g., several kilometers in radius)and may allow unrestricted access by UEs with service subscription. Apico cell may cover a relatively small geographic area and may allowunrestricted access by UEs with service subscription. A femto cell maycover a relatively small geographic area (e.g., a home) and may allowrestricted access by UEs having association with the femto cell (e.g.,UEs in a Closed Subscriber Group (CSG), UEs for users in the home,etc.). An eNB for a macro cell may be referred to as a macro eNB. An eNBfor a pico cell may be referred to as a pico eNB. An eNB for a femtocell may be referred to as a femto eNB or a home eNB. In the exampleshown in FIG. 1, eNB 110 may be a macro eNB for a macro cell 102, eNB112 may be a pico eNB for a pico cell 104, and eNBs 114 and 116 may befemto eNBs for femto cells 106 and 108, respectively. An eNB may supportone or multiple (e.g., three) cells.

Wireless network 100 may also include relay stations. A relay station isa station that receives a transmission of data and/or other informationfrom an upstream station (e.g., an eNB or a UE) and sends a transmissionof the data and/or other information to a downstream station (e.g., a UEor an eNB). A relay station may also be a UE that relays transmissionsfor other UEs. In the example shown in FIG. 1, a relay station 118 maycommunicate with macro eNB 110 and a UE 128 in order to facilitatecommunication between eNB 110 and UE 128. A relay station may also bereferred to as a relay eNB, a relay, etc.

Wireless network 100 may be a heterogeneous network that includes eNBsof different types, e.g., macro eNBs, pico eNBs, femto eNBs, relays,etc. These different types of eNBs may have different transmit powerlevels, different coverage areas, and different impact on interferencein wireless network 100. For example, macro eNBs may have a hightransmit power level (e.g., 20 Watts) whereas pico eNBs, femto eNBs andrelays may have a lower transmit power level (e.g., 1 Watt).

Wireless network 100 may support synchronous operation. For synchronousoperation, the eNBs may have similar frame timing, and transmissionsfrom different eNBs may be approximately aligned in time. Synchronousoperation may support certain transmission features, as described below.

A network controller 130 may couple to a set of eNBs and may providecoordination and control for these eNBs. Network controller 130 maycommunicate with the eNBs via a backhaul. The eNBs may also communicatewith one another, e.g., directly or indirectly via a wireless orwireline backhaul.

UEs 122, 124 and 128 may be dispersed throughout wireless network 100,and each UE may be stationary or mobile. A UE may also be referred to asa terminal, a mobile station, a subscriber unit, a station, etc. A UEmay be a cellular phone, a personal digital assistant (PDA), a wirelessmodem, a wireless communication device, a handheld device, a laptopcomputer, a cordless phone, a wireless local loop (WLL) station, etc. AUE may be able to communicate with macro eNBs, pico eNBs, femto eNBs,relays, etc. In FIG. 1, a solid line with double arrows indicatesdesired transmissions between a UE and a serving eNB, which is an eNBdesignated to serve the UE on the downlink and/or uplink. A dashed linewith double arrows indicates interfering transmissions between a UE andan eNB.

LTE utilizes orthogonal frequency division multiplexing (OFDM) on thedownlink and single-carrier frequency division multiplexing (SC-FDM) onthe uplink. OFDM and SC-FDM partition the system bandwidth into multiple(K) orthogonal subcarriers, which are also commonly referred to astones, bins, etc. Each subcarrier may be modulated with data. Ingeneral, modulation symbols are sent in the frequency domain with OFDMand in the time domain with SC-FDM. The spacing between adjacentsubcarriers may be fixed, and the total number of subcarriers (K) may bedependent on the system bandwidth. For example, K may be equal to 128,256, 512, 1024 or 2048 for system bandwidth of 1.25, 2.5, 5, 10 or 20megahertz (MHz), respectively. The system bandwidth may also bepartitioned into subbands. For example, a subband may cover 1.08 MHz,and there may be 1, 2, 4, 8 or 16 subbands for system bandwidth of 1.25,2.5, 5, 10 or 20 MHz, respectively.

FIG. 2 shows a frame structure used in LTE. The transmission timelinefor the downlink may be partitioned into units of radio frames. Eachradio frame may have a predetermined duration (e.g., 10 milliseconds(ms)) and may be partitioned into 10 subframes with indices of 0 through9. Each subframe may include two slots. Each radio frame may thusinclude 20 slots with indices of 0 through 19. Each slot may include Lsymbol periods, e.g., L=7 symbol periods for a normal cyclic prefix (asshown in FIG. 2) or L=6 symbol periods for an extended cyclic prefix.The 2L symbol periods in each subframe may be assigned indices of 0through 2L−1.

The available time frequency resources may be partitioned into resourceblocks. Each resource block may cover N subcarriers (e.g., 12subcarriers) in one slot and may include a number of resource elements.Each resource element may cover one subcarrier in one symbol period andmay be used to send one modulation symbol, which may be a real orcomplex value. An eNB may transmit one OFDM symbol in each symbolperiod. Each OFDM symbol may include modulation symbols on subcarriersused for transmission and zero symbols with signal value of zero on theremaining subcarriers.

In LTE, an eNB may send a primary synchronization signal (PSS) and asecondary synchronization signal (SSS) in the center 1.08 MHz of thesystem bandwidth for each cell in the eNB. The primary and secondarysynchronization signals may be sent in symbol periods 6 and 5,respectively, in each of subframes 0 and 5 of each radio frame with thenormal cyclic prefix, as shown in FIG. 2. The synchronization signalsmay be used by UEs for cell search and acquisition. The eNB may send aPhysical Broadcast Channel (PBCH) in symbol periods 0 to 3 in slot 1 ofsubframe 0 in certain radio frames. The PBCH may carry certain systeminformation.

The eNB may send a Physical Control Format Indicator Channel (PCFICH) inthe first symbol period of each subframe, as shown in FIG. 2. The PCFICHmay convey the number of symbol periods (M) used for control channels ina subframe, where M may be equal to 1, 2 or 3 and may change fromsubframe to subframe. M may also be equal to 4 for a small systembandwidth, e.g., with less than 10 resource blocks. The eNB may send aPhysical HARQ Indicator Channel (PHICH) and a Physical Downlink ControlChannel (PDCCH) in the first M symbol periods of each subframe (notshown in FIG. 2). The PHICH may carry information to support hybridautomatic retransmission (HARQ). The PDCCH may carry information onresource allocation for UEs and control information for downlinkchannels. The first M OFDM symbols of the subframe may also be referredto as TDM control symbols. A TDM control symbol may be an OFDM symbolcarrying control information. The eNB may send a Physical DownlinkShared Channel (PDSCH) in the remaining symbol periods of each subframe.The PDSCH may carry data for UEs scheduled for data transmission on thedownlink. The various signals and channels in LTE are described in 3GPPTS 36.211, entitled “Evolved Universal Terrestrial Radio Access(E-UTRA); Physical Channels and Modulation,” which is publiclyavailable.

LTE supports transmission of unicast information to specific UEs. LTEalso supports transmission of broadcast information to all UEs andmulticast information to a group of UEs. A multicast/broadcasttransmission may also be referred to as an MBSFN transmission. Asubframe used for sending unicast information may be referred to as aregular subframe. A subframe used for sending multicast and/or broadcastinformation may be referred to as an MBSFN subframe.

FIG. 3 shows two exemplary regular subframe formats 310 and 320 that maybe used to send unicast information to specific UEs on the downlink. Forthe normal cyclic prefix in LTE, the left slot includes seven symbolperiods 0 through 6, and the right slot includes seven symbol periods 7through 13.

Subframe format 310 may be used by an eNB equipped with two antennas. Acell-specific reference signal may be sent in symbol periods 0, 4, 7 and11 and may be used by UEs for channel estimation. A reference signal isa signal that is known a priori by a transmitter and a receiver and mayalso be referred to as pilot. A cell-specific reference signal is areference signal that is specific for a cell, e.g., generated with oneor more symbol sequences determined based on a cell identity (ID). Forsimplicity, a cell-specific reference signal may be referred to assimply a reference signal. In FIG. 3, for a given resource element withlabel R_(i), a reference symbol may be sent on that resource elementfrom antenna i, and no symbols may be sent on that resource element fromother antennas. Subframe format 320 may be used by an eNB equipped withfour antennas. A reference signal may be sent in symbol periods 0, 1, 4,7, 8 and 11.

In the example shown in FIG. 3, three TDM control symbols are sent in aregular subframe with M=3. The PCFICH may be sent in symbol period 0,and the PDCCH and PHICH may be sent in symbol periods 0 to 2. The PDSCHmay be sent in the remaining symbol periods 3 to 13 of the subframe.

FIG. 4 shows two exemplary MBSFN subframe formats 410 and 420 that maybe used to send broadcast/multicast information to UEs on the downlink.Subframe format 410 may be used by an eNB equipped with two antennas. Areference signal may be sent in symbol period 0. For the example shownin FIG. 4, M=1 and one TDM control symbol may be sent in the MBSFNsubframe. Subframe format 420 may be used by an eNB equipped with fourantennas. A reference signal may be sent in symbol periods 0 and 1. Forthe example shown in FIG. 4, M=2 and two TDM control symbols may be sentin the MBSFN subframe.

In general, the PCFICH may be sent in symbol period 0 of an MBSFNsubframe, and the PDCCH and PHICH may be sent in symbol periods 0 toM−1. Broadcast/multicast information may be sent in symbol periods Mthrough 13 of the MBSFN subframe. Alternatively, no transmissions may besent in symbol periods M through 13.

FIGS. 3 and 4 show some subframe formats that may be used for thedownlink. Other subframe formats may also be used, e.g., for more thantwo antennas at the eNB.

An eNB or a relay may operate in a regular mode, an MBSFN mode, and/orother operating modes. The eNB or relay may switch mode from subframe tosubframe, or at a slower rate. In the regular mode, the eNB or relay maytransmit using a regular subframe format, e.g., as shown in FIG. 3. Theregular mode may be associated with certain characteristics such as aconfigurable number of TDM control symbols, the reference signal beingsent from each antenna in two or more symbol periods of a subframe, etc.In the MBSFN mode, the eNB or relay may transmit using an MBSFN subframeformat, e.g., as shown in FIG. 4. The MBSFN mode may be associated withcertain characteristics such as a minimum number of TDM control symbols,the reference signal being sent from each antenna in one symbol periodof a subframe, etc. The eNB or relay may transmit control informationand reference signal in fewer symbol periods in the MBSFN mode than theregular mode, e.g., as shown in FIGS. 3 and 4. The eNB or relay may alsotransmit fewer TDM control symbols in the MBSFN mode than the regularmode. The MBSFN mode may thus be desirable under certain operatingscenarios, as described below.

A UE may be within the coverage of multiple eNBs. One of these eNBs maybe selected to serve the UE. The serving eNB may be selected based onvarious criteria such as received power, pathloss, signal-to-noise ratio(SNR), etc.

A UE may operate in a dominant interference scenario in which the UE mayobserve high interference from one or more interfering eNBs. A dominantinterference scenario may occur due to restricted association. Forexample, in FIG. 1, UE 124 may be close to femto eNB 114 and may havehigh received power for eNB 114. However, UE 124 may not be able toaccess femto eNB 114 due to restricted association and may then connectto macro eNB 110 with lower received power (as shown in FIG. 1) or tofemto eNB 116 also with lower received power (not shown in FIG. 1). UE124 may then observe high interference from femto eNB 114 on thedownlink and may also cause high interference to eNB 114 on the uplink.

A dominant interference scenario may also occur due to range extension,which is a scenario in which a UE connects to an eNB with lower pathlossand possibly lower SNR among all eNBs detected by the UE. For example,in FIG. 1, UE 122 may detect macro eNB 110 and pico eNB 112 and may havelower received power for pico eNB 112 than macro eNB 110. Nevertheless,it may be desirable for UE 122 to connect to pico eNB 112 if thepathloss for pico eNB 112 is lower than the pathloss for macro eNB 110.This may result in less interference to the wireless network for a givendata rate for UE 122.

In an aspect, communication in a dominant interference scenario may besupported by reserving subframes for a weaker eNB observing highinterference from a strong interfering eNB. A UE can then communicatewith the weaker eNB in the reserved subframes in the presence of thestrong interfering eNB. An eNB may be classified as a “weak” eNB or a“strong” eNB based on the received power of the eNB at a UE (and notbased on the transmit power level of the eNB). Furthermore, differenteNBs may send their synchronization signals such that interference froma dominant interferer can be avoided.

In one design, eNBs and relays may be arranged into different groups.Each group may include eNBs and/or relays that are not dominantinterferers of one another. For example, one group may include macroeNBs, another group may include pico eNBs and relays, and one or moregroups may include femto eNBs. Relays may have a similar transmit powerlevel as pico eNBs and may thus be grouped with the pico eNBs. FemtoeNBs may be divided into multiple groups if they are dominantinterferers of one another. By having each group includes eNBs that arenot dominant interferers of one another, outage scenarios may be avoidedand the benefits of range extension may be realized.

In one design, different groups of eNBs may be associated with differentsubframe offsets. The timing of eNBs in different groups may be offsetfrom one another by an integer number of subframes. For example, whenmacro eNBs in a first group transmit subframe 0, pico eNBs in a secondgroup may transmit subframe 1, femto eNBs in a third group may transmitsubframe 2, etc. The use of subframe offset may allow eNBs and relays indifferent groups to transmit their synchronization signals such that UEscan detect these signals.

FIG. 5 shows an exemplary transmission timeline for four groups of eNBsand relay. A first group may include macro eNB 110, which may have itssubframe 0 starts at time T₀. A second group may include pico eNB 112and relay 118, which may have their subframe 0 starts one subframe aftertime T₀. A third group may include femto eNB 114, which may have itssubframe 0 starts two subframes after time T₀. A fourth group mayinclude femto eNB 116, which may have its subframe 0 starts threesubframes after T₀. In general, any number of groups may be formed, andeach group may include any number of eNBs and/or relays.

In one design, a strong interfering eNB may reserve or clear somesubframes for a weaker eNB to allow the weaker eNB to communicate withits UEs. The interfering eNB may transmit as little as possible in thereserved subframes in order to reduce interference to the weaker eNB. Inone design, the interfering eNB may configure the reserved subframes asMBSFN subframes. The interfering eNB may transmit only the PCFICH withM=1 and the reference signal in the first symbol period of each reservedsubframe and may transmit nothing in the remaining symbol periods of thesubframe, e.g., as shown in FIG. 4. In another design, the interferingeNB may operate in a 1-Tx mode with one transmit antenna or a 2-Tx modewith two transmit antennas. The interfering eNB may transmit the PCFICHwith M=1 and the reference signal in each reserved subframe, e.g., asshown in FIG. 3. In yet another design, the interfering eNB may transmitthe reference signal but may avoid transmitting the PCFICH in thereserved subframes in order to reduce interference to the weaker eNB.For the designs described above, the interfering eNB may avoidtransmitting other control channels, such as the PHICH and PDCCH, aswell as data in each reserved subframe. In yet another design, theinterfering eNB may transmit nothing in each reserved subframe in orderto avoid causing any interference to the weaker eNB. The interfering eNBmay also transmit in the reserved subframes in other manners. Theinterfering eNB may transmit the least number of modulation symbolsrequired by the LTE standard in each reserved subframe.

In the example shown in FIG. 5, macro eNB 110 reserves subframes 1 and 6for pico eNB 112 and transmits one TDM control symbol with M=1 for thePCFICH in each reserved subframe. Femto eNB 114 (femto eNB A) reservessubframes 3 and 8 for macro eNB 110, reserves subframes 4 and 9 for picoeNB 112, and reserves subframe 1 for femto eNB 116 (femto eNB B). FemtoeNB 114 transmits one TDM control symbol with M=1 for the PCFICH in eachreserved subframe. Femto eNB 116 reserves subframes 2 and 7 for macroeNB 110, reserves subframes 3 and 8 for pico eNB 112, and reservessubframe 9 for femto eNB 114. Femto eNB 116 transmits one TDM controlsymbol with M=1 for the PCFICH in each reserved subframe. As shown inFIG. 5, the subframes reserved for macro eNB 110 by femto eNBs 114 and116 are time aligned and allow the macro eNB to transmit in itssubframes 0 and 5 with little interference from the femto eNBs. Thesubframes reserved for pico eNB 112 by macro eNB 110 and femto eNBs 114and 116 are time aligned and allow the pico eNB to transmit in itssubframes 0 and 5 with little interference from the macro and femtoeNBs.

Referring back to FIG. 2, each eNB may transmit its synchronizationsignals in subframes 0 and 5 and may also transmit the PBCH in subframe0. A UE may search for the synchronization signals when detecting foreNBs and may receive the PBCH from each detected eNB in order tocommunicate with the eNB. To allow UEs to detect a weaker eNB, a stronginterfering eNB may reserve or clear subframes in which thesynchronization signals and the PBCH are transmitted by the weaker eNB.This clearing may be done for all subframes or only some subframes inwhich the synchronization signals and the PBCH are transmitted by theweaker eNB. The clearing should be done such that UEs can detect theweaker eNB in a reasonable amount of time.

Referring to the example shown in FIG. 5, subframes 0 and 5 of macro eNB110 are cleared by femto eNBs 114 and 116 to avoid interference to thesynchronization signals and the PBCH from the macro eNB. Subframes 0 and5 of pico eNB 112 are cleared by macro eNB 110 and femto eNBs 114 and116 to avoid interference to the synchronization signals and the PBCHfrom the pico eNB. Subframe 0 of femto eNB 114 is cleared by femto eNB116, and subframe 0 of femto eNB 116 is cleared by femto eNB 114.

In one design, the eNBs may communicate via the backhaul to negotiatereservation/clearing of subframes. In another design, a UE desiring tocommunicate with a weaker eNB may request an interfering eNB to reservesome subframes for the weaker eNB. In yet another design, a designatednetwork entity may decide reservation of subframes for the eNBs, e.g.,based on data requests sent by UEs to different eNBs and/or reports fromthe eNBs. For all designs, subframes may be reserved based on variouscriteria such as loading at the eNBs, the number of eNBs in thevicinity, the number of UEs within the coverage of each eNB, pilotmeasurement reports from the UEs, etc. For example, a macro eNB mayreserve a subframe to allow multiple pico eNBs and/or femto eNBs tocommunicate with their UEs, which may provide cell splitting gains.

Each eNB may transmit its reference signal on a set of subcarriersdetermined based on its cell ID. In one design, the cell ID space ofstrong interfering eNBs (such as macro eNBs) and weaker eNBs (such aspico eNBs) may be defined such that the reference signals of these eNBsare transmitted on different subcarriers and do not collide. Some eNBs(such as femto eNBs and relays) may be self-configuring. These eNBs mayselect their cell IDs such that their reference signals do not collidewith the reference signals of strong neighboring eNBs.

A UE may communicate with a weaker eNB in a reserved subframe and mayobserve high interference due to the PCFICH, the reference signal, andpossibly other transmissions from a strong interfering eNB. In onedesign, the UE may discard each TDM control symbol with highinterference from the interfering eNB and may process remaining TDMcontrol symbols. In another design, the UE may discard received symbolson subcarriers with high interference and may process remaining receivedsymbols. The UE may also process the received symbols and the TDMcontrol symbols in other manners.

The UE may obtain a channel estimate for the weaker eNB based on areference signal transmitted by the weaker eNB. The reference signal ofthe weaker eNB may be transmitted on different subcarriers and may notoverlap with the reference signal of the strong interfering eNB. In thiscase, the UE may derive a channel estimate for the weaker eNB based onthe reference signal from this eNB. If the reference signal of theweaker eNB collides with the reference signal of the interfering eNB,then the UE may perform channel estimation with interferencecancellation. The UE may estimate the interference due to the referencesignal from the interfering eNB based on known reference symbols sent bythis eNB and the known subcarriers on which the reference signal istransmitted. The UE may subtract the estimated interference from thereceived signal at the UE to remove the interference due to theinterfering eNB and may then derive a channel estimate for the weakereNB based on the interference-canceled signal. The UE may also performinterference cancellation for control channels (e.g., the PCFICH) fromthe interfering eNB that collide with the reference signal from theweaker eNB. The UE may decode each such control channel from theinterfering eNB, estimate the interference due to each decoded controlchannel, subtract the estimated interference from the received signal,and derive the channel estimate for the weaker eNB after subtracting theestimated interference. In general, the UE may perform interferencecancellation for any transmission from the interfering eNB which can bedecoded in order to improve channel estimation performance. The UE maydecode control channels (e.g., the PBCH, PHICH and PDCCH) as well as thedata channel (e.g., the PDSCH) from the weaker eNB based on the channelestimate.

The weaker eNB may send control information and data to the UE in asubframe reserved by the interfering eNB. The interfering eNB maytransmit only the first TDM control symbol in the subframe, e.g., asshown in FIG. 4. In this case, the UE may observe high interference ononly the first TDM control symbol and may observe no interference fromthe interfering eNB on the remaining TDM control symbols in thesubframe.

The weaker eNB may transmit control information in a manner tofacilitate reliable reception by the UE in the presence of theinterfering eNB. In one design, the weaker eNB may transmit three TDMcontrol symbols in a reserved subframe by setting M=3 for the PCFICH. Inanother design, the weaker eNB may transmit a predetermined number ofTDM control symbols in the reserved subframe. For both designs, the UEmay be aware of the number of TDM control symbols being transmitted bythe weaker eNB. The UE would not need to decode the PCFICH sent by theweaker eNB in the first TDM control symbol, which may observe highinterference from the interfering eNB.

The weaker eNB may send three transmissions of the PHICH in three TDMcontrol symbols, one PHICH transmission in each TDM control symbol. TheUE may decode the PHICH based on the two PHICH transmissions sent in thesecond and third TDM control symbols, which may observe no interferencefrom the interfering eNB. The UE may decode the PHICH based further on aportion of the PHICH transmission sent on subcarriers not used by theinterfering eNB in the first TDM control symbol.

The weaker eNB may also send the PDCCH in three TDM control symbols. Theweaker eNB may send the PDCCH to the UE such that adverse impact due tointerference from the interfering eNB can be reduced. For example, theweaker eNB may send the PDCCH in TDM control symbols withoutinterference from the interfering eNB, on subcarriers not used by theinterfering eNB, etc.

The weaker eNB may be aware of the interference due to the interferingeNB and may transmit the control information to mitigate the adverseeffects of the interference. In one design, the weaker eNB may scale thetransmit power of the PHICH, the PDCCH, and/or other control channels toobtain the desirable performance. The power scaling may account for theloss of part of the control information due to puncturing by the highinterference from the interfering eNB.

The UE may decode the control channels (e.g., the PHICH and PDCCH) fromthe weaker eNB with knowledge that some modulation symbols in the firstTDM control symbol may be lost or punctured due to high interferencefrom the interfering eNB. In one design, the UE may discard receivedsymbols with high interference from the interfering eNB and may decodethe remaining received symbols. The discarded symbols may be replacedwith erasures and given neutral weight in the decoding process. Inanother design, the UE may perform decoding with interferencecancellation for the control channels. The UE may estimate theinterference due to the interfering eNB in the TDM control symbols,remove the estimated interference from the received symbols, and use thereceived symbols after interference cancellation to decode the controlchannels.

The UE may decode the data channel (e.g., PDSCH) from the weaker eNB,possibly with knowledge that some modulation symbols may be punctureddue to high interference from the interfering eNB. In one design, the UEmay discard received symbols with high interference from the interferingeNB and may decode the remaining received symbols to recover the datasent by the weaker eNB. In another design, the UE may perform decodingwith interference cancellation for the data channel.

The UE may also decode the control and data channels from the weaker eNBbased on other techniques to improve performance in the presence of highinterference from the interfering eNB. For example, the UE may performdetection and/or decoding by taking into account high interference oncertain received symbols.

The techniques described herein may be used to support operation byrelays, e.g., relay 118. In the downlink direction, relay 118 mayreceive data and control information from macro eNB 110 and mayretransmit the data and control information to UE 128. In the uplinkdirection, relay 118 may receive data and control information from UE128 and may retransmit the data and control information to macro eNB110. Relay 118 may appear like a UE to macro eNB 110 and like an eNB toUE 128. The link between macro eNB 110 and relay 118 may be referred toas a backhaul link, and the link between relay 118 and UE 128 may bereferred to as a relay link.

Relay 118 typically cannot transmit and receive at the same time on thesame frequency channel or bandwidth. In the downlink direction, relay118 may designate some subframes as backhaul downlink subframes in whichit will listen to macro eNB 110 and some subframes as relay downlinksubframes in which it will transmit to UEs. In the uplink direction,relay 118 may designate some subframes as relay uplink subframes inwhich it will listen to the UEs and some subframes as backhaul uplinksubframes in which it will transmit to macro eNB 110. In the exampleshown in FIG. 5, in the downlink direction, relay 118 may transmit toits UEs in subframes 0 and 5, which may be cleared by macro eNB 110, andmay listen to macro eNB 110 in subframes 1, 2, 3, 4 and 9. The subframesfor the uplink direction are not shown in FIG. 5.

In a range extension scenario, macro eNB 110 may be a strong interferingeNB to UEs communicating with relay 118 as well as new UEs that can beserved by relay 118. For the relay downlink subframes in which relay 118transmits to the UEs, the timing of relay 118 may be shifted by aninteger number of subframes (e.g., by one subframe in FIG. 5) from thetiming of macro eNB 110. Macro eNB 110 may clear some subframes (e.g.,subframes 1 and 6 in FIG. 5) for relay 118. Relay 118 may transmit itssynchronization signals and the PBCH in relay downlink subframes thatcoincide with the subframes reserved by macro eNB 110. UEs can detectthe synchronization signals from relay 118. The UEs may be aware ofsymbols punctured by macro eNB 110 and may make use of this informationto decode the control channels from relay 118, as described above.

For the backhaul downlink subframes, relay 118 may desire to only listento macro eNB 110 and may not desire to transmit anything to its UEs inthese subframes. However, since relay 118 is an eNB to its UEs, relay118 may be expected to transmit some signals to its UEs in the backhauldownlink subframes. In one design, relay 118 may operate in the MBSFNmode for the backhaul downlink subframes. In the MBSFN mode, relay 118may transmit only in the first symbol period of a backhaul downlinksubframe and may listen to macro eNB 110 in the remaining symbol periodsof the subframe. In the example shown in FIG. 5, relay 118 transmits inonly the first symbol period of subframes 1, 2, 3, 4 and 9, which aresubframes in which relay 118 listens to macro eNB 110.

In one design, macro eNB 110 may set the PCFICH to a predetermined value(e.g., M=3) in subframes in which macro eNB 110 transmits to relay 118(e.g., subframes 0 and 5 of macro eNB 110 in FIG. 5). Relay 118 may knowthe predetermined value of the PCFICH from macro eNB 110 and may skipdecoding the PCFICH. Relay 118 may transmit the PCFICH to its UEs in thefirst symbol period and may skip decoding the PCFICH sent by macro eNB110 in the same symbol period. Macro eNB 110 may send threetransmissions of the PHICH, one transmission in each TDM control symbol.Relay 118 may decode the PHICH from macro eNB 110 based on the PHICHtransmissions in the second and third TDM control symbols. Macro eNB 110may also send the PDCCH such that all or most of a PDCCH transmissionfor relay 118 is sent in the second and third TDM control symbols. Relay118 may decode the PDCCH based on the portion of the PDCCH transmissionreceived in the second and third TDM control symbols. Macro eNB 110 mayboost the transmit power of the control channels (e.g., the PHICH and/orPDCCH) intended for relay 118 to improve reception of the controlchannels by relay 118 based on the part sent in the second and third TDMcontrol symbols. Macro eNB 110 may also skip transmitting controlinformation in the first TDM control symbol to relay 118. Macro eNB 110may send data to relay 118 in symbol periods 3 through 13. Relay 118 mayrecover the data in the normal manner.

Relay 118 may be unable to receive the reference signal from macro eNB110 in symbol period 0. Relay 118 may derive a channel estimate formacro eNB 110 based on the reference signal that relay 118 can receivefrom macro eNB 110. When scheduling relay 118, macro eNB 110 may makeuse of the information about which subframes are likely to have betterchannel estimates by relay 118. For example, relay 118 may listen tomacro eNB 110 in two contiguous subframes. In this case, the channelestimate for the first subframe may be worse than the channel estimatefor the second subframe since the channel estimate for the firstsubframe may be extrapolated whereas the channel estimate for the secondsubframe may be interpolated and may have more reference symbols aroundit. Macro eNB 110 may then send data to relay 118 in the secondsubframe, if possible.

Relay 118 may not be able to operate in the MBSFN mode in its subframes0 and 5, which carry the synchronization signals. In one design, relay118 may skip listening to macro eNB 110 in subframes 0 and 5 of relay118, even if these subframes are designated as backhaul downlinksubframes, and may instead transmit to its UEs. In another design, relay118 may skip transmitting to its UEs in subframes 0 and 5, even if thesesubframes are designated as relay downlink subframes, and may insteadlisten to macro eNB 110. Relay 118 may also perform a combination ofboth and may transmit to its UEs in some of subframes 0 and 5 and maylisten to macro eNB 110 in some other subframes 0 and 5.

In the uplink direction, relay 118 may operate in a similar manner as aUE in the backhaul uplink subframes in which relay 118 transmits dataand control information to macro eNB 110. Relay 118 may operate in asimilar manner as an eNB in the relay uplink subframes in which relay118 listens for data and control information from UE 128. A scheduler atmacro eNB 110 and/or a scheduler at relay 118 may ensure that the uplinkof relay 118 and the uplink of UEs served by relay 118 are scheduledappropriately.

FIG. 6 shows a design of a process 600 for mitigating interference in awireless communication network. Process 600 may be performed by a UE, abase station/eNB, a relay station, or some other entity. A first stationcausing high interference to or observing high interference from asecond station in a heterogeneous network may be identified (block 612).The heterogeneous network may comprise base stations of at least twodifferent transmit power levels and/or different association types.Interference due to a first reference signal from the first station maybe mitigated by canceling the interference at the second station, orinterference to the first reference signal may be mitigated by selectingdifferent resources for sending a second reference signal by the secondstation to avoid collision with the first reference signal (block 614).

In one design, the first station may be a base station or a relaystation, and the second station may be a UE. For block 614, theinterference due to the first reference signal may be canceled at theUE. In one design, the interference due to the first reference signalmay be estimated and subtracted from a received signal at the UE toobtain an interference-canceled signal. The interference-canceled signalmay then be processed to obtain a channel estimate for a base station ora relay station with which the UE is in communication. Theinterference-canceled signal may also be processed to obtain data and/orcontrol information sent by the base station or the relay station to theUE.

In another design, the first and second stations may comprise (i) amacro base station and a pico base station, respectively, (ii) two femtobase stations, or (iii) some other combination of macro, pico, and femtobase stations and relay station. For block 614, first resources used tosend the first reference signal by the first station may be determined.A cell ID associated with second resources for sending the secondreference signal may be selected such that the second resources aredifferent from the first resources. The first resources may comprise afirst set of subcarriers, and the second resources may comprise a secondset of subcarriers, which may be different from the first set ofsubcarriers. The second reference signal may be sent on the secondresources by the second station and may then avoid collision with thefirst reference signal. A primary synchronization signal and a secondarysynchronization signal may be generated based on the selected cell IDand may be sent by the second station in designated subframes, e.g.,subframes 0 and 5.

FIG. 7 shows a design of an apparatus 700 for mitigating interference.Apparatus 700 includes a module 712 to identify a first station causinghigh interference to or observing high interference from a secondstation in a heterogeneous network, and a module 714 to mitigateinterference due to a first reference signal from the first station bycanceling the interference at the second station or mitigateinterference to the first reference signal by selecting differentresources for sending a second reference signal by the second station toavoid collision with the first reference signal

FIG. 8 shows a design of a process 800 for operating a relay station ina wireless communication network. The relay station may determinesubframes in which it listens to a macro base station (block 812). Therelay station may transmit in an MBSFN mode in the subframes in which itlistens to the macro base station (block 814). The relay station mayalso determine subframes in which it transmits to UEs (block 816). Therelay station may transmit in a regular mode in the subframes in whichit transmits to the UEs (block 818).

The relay station may send a reference signal in fewer symbol periods ina given subframe in the MBSFN mode than the regular mode. In one design,the relay station may transmit the reference signal from each antenna inone symbol period of each subframe in which the relay station listens tothe macro base station in the MBSFN mode, e.g., as shown in FIG. 4. Therelay station may transmit the reference signal from each antenna inmultiple symbol periods of each subframe in which the relay stationtransmits to the UEs in the regular mode, e.g., as shown in FIG. 3. Inone design, the relay station may transmit the reference signal in onlythe first symbol period or only the first two symbol periods of eachsubframe in which the relay station listens to the macro base station inthe MBSFN mode. The relay station may transmit the reference signal inmore symbol periods across each subframe in which the relay stationtransmits to the UEs in the regular mode. The relay station may alsotransmit the reference signal in other manners in the MBSFN mode and theregular mode.

In one design of block 814, the relay station may transmit a single TDMcontrol symbol and may transmit no data in each subframe in which itlistens to the macro base station. The relay station may receive amaximum number of (e.g., three) TDM control symbols from the macro basestation in each subframe in which the macro base station transmits tothe relay station. The relay station may decode at least one controlchannel (e.g., the PHICH and PDCCH) from the macro base station based onthe second and third TDM control symbols.

FIG. 9 shows a design of an apparatus 900 for operating a relay station.Apparatus 900 includes a module 912 to determine subframes in which arelay station is listening to a macro base station, a module 914 totransmit in an MBSFN mode by the relay station in the subframes in whichthe relay station is listening to the macro base station, a module 916to determine subframes in which the relay station is transmitting toUEs, and a module 918 to transmit in the regular mode by the relaystation in the subframes in which the relay station is transmitting tothe UEs.

FIG. 10 shows a design of a process 1000 for transmitting controlinformation in a wireless communication network. Process 1000 may beperformed by a first station, which may be a base station/eNB, a relaystation, or some other entity. The first station may identify a stronginterfering station to the first station (block 1012). The first stationmay determine a first number of TDM control symbols being transmitted bythe strong interfering station in a subframe (block 1014). The firststation may transmit a second number of TDM control symbols in thesubframe, with the second number of TDM control symbols being more thanthe first number of TDM control symbols (block 1016). The second numberof TDM control symbols may be the maximum number of TDM control symbolsallowed for the first station and may comprise three TDM controlsymbols.

The first station and the strong interfering station may have differenttransmit power levels. In one design, the first station may be a picobase station, and the interfering station may be a macro base station.In another design, the first station may be a macro base station, andthe interfering station may be a femto base station, or vice versa. Inyet another design, the first station may be a femto base station, andthe interfering station may be another femto base station. The firststation and the strong interfering station may also be some othercombination of macro base station, pico base station, femto basestation, relay station, etc.

In one design, the first station may transmit a control channel (e.g.,the PCFICH) indicating the second number of TDM control symbols beingtransmitted in the subframe if the strong interfering station is notpresent. The first station may not transmit the control channel if thestrong interfering station is present. In this case, a predeterminedvalue may be assumed for the second number of TDM control symbols.

In one design of block 1016, the first station may transmit a controlchannel (e.g., the PHICH or PDCCH) in a first TDM control symbol at afirst transmit power level. The first station may transmit the controlchannel in at least one additional TDM control symbol at a secondtransmit power level, which may be higher than the first transmit powerlevel. In another design of block 1016, the first station may transmit acontrol channel (e.g., the PHICH or PDCCH) in the second number of TDMcontrol symbols on resource elements selected to avoid or reducecollision with a reference signal from the strong interfering station.The first station may also transmit the second number of TDM controlsymbols in other manners to mitigate the effects of interference fromthe strong interfering station.

FIG. 11 shows a design of an apparatus 1100 for transmitting controlinformation. Apparatus 1100 includes a module 1112 to identify a stronginterfering station to a first station, a module 1114 to determine afirst number of TDM control symbols being transmitted by the stronginterfering station in a subframe, and a module 1116 to transmit asecond number of TDM control symbols by the first station in thesubframe, the second number of TDM control symbols being more than thefirst number of TDM control symbols.

The modules in FIGS. 7, 9 and 11 may comprise processors, electronicsdevices, hardware devices, electronics components, logical circuits,memories, software codes, firmware codes, etc., or any combinationthereof.

FIG. 12 shows a block diagram of a design of a station 110 x and a UE120. Station 110 x may be macro base station 110, pico base station 112,femto base station 114 or 116, or relay station 118 in FIG. 1. UE 120may be any of the UEs in FIG. 1. Station 110 x may be equipped with Tantennas 1234 a through 1234 t, and UE 120 may be equipped with Rantennas 1252 a through 1252 r, where in general T≧1 and R≧1.

At station 110 x, a transmit processor 1220 may receive data from a datasource 1212 and control information from a controller/processor 1240.The control information may be for the PBCH, PCFICH, PHICH, PDCCH, etc.The data may be for the PDSCH, etc. Processor 1220 may process (e.g.,encode and symbol map) the data and control information to obtain datasymbols and control symbols, respectively. Processor 1220 may alsogenerate reference symbols, e.g., for the PSS, SSS, and cell-specificreference signal. A transmit (TX) multiple-input multiple-output (MIMO)processor 1230 may perform spatial processing (e.g., precoding) on thedata symbols, the control symbols, and/or the reference symbols, ifapplicable, and may provide T output symbol streams to T modulators(MODs) 1232 a through 1232 t. Each modulator 1232 may process arespective output symbol stream (e.g., for OFDM, etc.) to obtain anoutput sample stream. Each modulator 1232 may further process (e.g.,convert to analog, amplify, filter, and upconvert) the output samplestream to obtain a downlink signal. T downlink signals from modulators1232 a through 1232 t may be transmitted via T antennas 1234 a through1234 t, respectively.

At UE 120, antennas 1252 a through 1252 r may receive the downlinksignals from station 110 x and may provide received signals todemodulators (DEMODs) 1254 a through 1254 r, respectively. Eachdemodulator 1254 may condition (e.g., filter, amplify, downconvert, anddigitize) a respective received signal to obtain input samples. Eachdemodulator 1254 may further process the input samples (e.g., for OFDM,etc.) to obtain received symbols. A MIMO detector 1256 may obtainreceived symbols from all R demodulators 1254 a through 1254 r, performMIMO detection on the received symbols if applicable, and providedetected symbols. A receive processor 1258 may process (e.g.,demodulate, deinterleave, and decode) the detected symbols, providedecoded data for UE 120 to a data sink 1260, and provide decoded controlinformation to a controller/processor 1280.

On the uplink, at UE 120, a transmit processor 1264 may receive andprocess data (e.g., for the PUSCH) from a data source 1262 and controlinformation (e.g., for the PUCCH) from controller/processor 1280.Processor 1264 may also generate reference symbols for a referencesignal. The symbols from transmit processor 1264 may be precoded by a TXMIMO processor 1266 if applicable, further processed by modulators 1254a through 1254 r (e.g., for SC-FDM, etc.), and transmitted to station110 x. At station 110 x, the uplink signals from UE 120 may be receivedby antennas 1234, processed by demodulators 1232, detected by a MIMOdetector 1236 if applicable, and further processed by a receiveprocessor 1238 to obtain decoded data and control information sent by UE120. Processor 1238 may provide the decoded data to a data sink 1239 andthe decoded control information to controller/processor 1240.

Controllers/processors 1240 and 1280 may direct the operation at station110 x and UE 120, respectively. Processor 1240 and/or other processorsand modules at station 110 x may perform or direct process 600 in FIG.6, process 800 in FIG. 8, process 1000 in FIG. 10, and/or otherprocesses for the techniques described herein. Processor 1280 and/orother processors and modules at UE 120 may perform or direct process 600in FIG. 6 and/or other processes for the techniques described herein.Memories 1242 and 1282 may store data and program codes for station 110x and UE 120, respectively. A scheduler 1244 may schedule UEs for datatransmission on the downlink and/or uplink.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for wireless communication, comprising:identifying, at a second station, a first station causing highinterference to the second station in a heterogeneous network comprisingbase stations of at least two different transmit power levels, whereinthe first station is a base station or a relay station and the secondstation is a user equipment (UE); and mitigating interference, at thesecond station, due to a first reference signal from the first stationby canceling the interference due to the first reference signal at theUE; wherein the canceling of the interference comprises: estimating theinterference due to the first reference signal at the UE, subtractingthe estimated interference from a signal received at the UE from a thirdstation to obtain an interference-canceled signal, and processing theinterference-canceled signal to obtain a channel estimate for the thirdstation.
 2. The method of claim 1, wherein the first station is a macrobase station.
 3. The method of claim 1, wherein the first station is afemto base station.
 4. A method for wireless communication, comprising:identifying, at a second station, a first station causing highinterference to the second station or observing high interference fromthe second station in a heterogeneous network comprising base stationsof at least two different transmit power levels; and mitigatinginterference, at the second station, associated with a first referencesignal from the first station including mitigating interference to thefirst reference signal by clearing resources associated with the firstreference signal and selecting different resources for sending a secondreference signal by the second station to avoid collision with the firstreference signal; wherein the mitigating interference to the firstreference signal comprises determining first resources used to send thefirst reference signal by the first station, selecting a cell identity(ID) associated with second resources for sending the second referencesignal, the second resources being different from the first resources,sending the second reference signal on the second resources by thesecond station, generating a primary synchronization signal and asecondary synchronization signal based on the selected cell ID, andsending the primary and secondary synchronization signals in designatedsubframes by the second station.
 5. The method of claim 1, wherein thefirst resources comprise a first set of subcarriers, and wherein thesecond resources comprise a second set of subcarriers different from thefirst set of subcarriers.
 6. An apparatus for wireless communication,comprising: means for identifying, at a second station, a first stationcausing high interference to the second station in a heterogeneousnetwork comprising base stations of at least two different transmitpower levels, wherein the first station is a base station or a relaystation and the second station is a user equipment (UE); and means formitigating interference, at the second station, due to a first referencesignal from the first station by canceling the interference due to thefirst reference signal at the UE; wherein the canceling of theinterference by the means for mitigating interference comprises:estimating the interference due to the first reference signal at the UE,subtracting the estimated interference from a signal received at the UEfrom a third station to obtain an interference-canceled signal, andprocessing the interference-canceled signal to obtain a channel estimatefor the third station.
 7. An apparatus for wireless communication,comprising: means for identifying, at a second station, a first stationcausing high interference to the second station or observing highinterference from the second station in a heterogeneous networkcomprising base stations of at least two different transmit powerlevels; and means for mitigating interference, at the second station,associated with a first reference signal from the first station bymitigating interference to the first reference signal by clearingresources associated with the first reference signal and selectingdifferent resources for sending a second reference signal by the secondstation to avoid collision with the first reference signal; wherein themeans for—mitigating interference mitigates interference to the firstreference signal by: determining first resources used to send the firstreference signal by the first station, selecting a cell identity (ID)associated with second resources for sending the second referencesignal, the second resources being different from the first resources,sending the second reference signal on the second resources by thesecond station, and generating a primary synchronization signal and asecondary synchronization signal based on the selected cell ID; andsending the primary and secondary synchronization signals in designatedsubframes by the second station.
 8. An apparatus for wirelesscommunication, comprising: a memory; and at least one processor coupledto the memory and configured to: identify, at a second station, a firststation causing high interference to the second station in aheterogeneous network comprising base stations of at least two differenttransmit power levels, wherein the first station is a base station or arelay station and the second station is a user equipment (UE); andmitigate interference, at the second station, due to a first referencesignal from the first station by canceling the interference due to thefirst reference signal at the UE; wherein the canceling of theinterference comprises: estimating the interference due to the firstreference signal at the UE, subtracting the estimated interference froma signal received at the UE from a third station to obtain aninterference-canceled signal, and processing the interference-canceledsignal to obtain a channel estimate for the third station.
 9. Anapparatus for wireless communication, comprising: a memory; and at leastone processor coupled to the memory and configured to: identify, at asecond station, a first station causing high interference to the secondstation or observing high interference from the second station in aheterogeneous network comprising base stations of at least two differenttransmit power levels; and mitigate interference, at the second station,associated with a first reference signal from the first station,including mitigating interference to the first reference signal byclearing resources associated with the first reference signal andselecting different resources for sending a second reference signal bythe second station to avoid collision with the first reference signal;wherein mitigating interference to the first reference signal comprisesdetermining first resources used to send the first reference signal bythe first station, selecting a cell identity (ID) associated with secondresources for sending the second reference signal, the second resourcesbeing different from the first resources, and to sending the secondreference signal on the second resources by the second station;generating a primary synchronization signal and a secondarysynchronization signal based on the selected cell ID, and sending theprimary and secondary synchronization signals in designated subframes bythe second station.
 10. A non-transitory computer-readable mediumstoring computer executable code for wireless communication, comprisingcode for causing at least one computer to: identify, at a secondstation, a first station causing high interference to the second stationin a heterogeneous network comprising base stations of at least twodifferent transmit power levels, wherein the first station is a basestation or a relay station and the second station is a user equipment(UE); and mitigate interference, at the second station, due to a firstreference signal from the first station by canceling the interferencedue to the first reference signal at the UE; wherein the canceling ofthe interference comprises: estimating the interference due to the firstreference signal at the UE, subtracting the estimated interference froma signal received at the UE from a third station to obtain aninterference-canceled signal, and processing the interference-canceledsignal to obtain a channel estimate for the third station.
 11. Anon-transitory computer-readable medium storing computer executable codefor wireless communication, comprising code for causing at least onecomputer to: identify, at a second station, a first station causing highinterference to the second station or observing high interference fromthe second station in a heterogeneous network comprising base stationsof at least two different transmit power levels; and mitigateinterference, at the second station, associated with a first referencesignal from the first station, including mitigating interference to thefirst reference signal by clearing resources associated with the firstreference signal and selecting different resources for sending a secondreference signal by the second station to avoid collision with the firstreference signal; wherein the mitigating interference due to a firstreference signal comprises: determining first resources used to send thefirst reference signal by the first station, selecting a cell identity(ID) associated with second resources for sending the second referencesignal, the second resources being different from the first resources,sending the second reference signal on the second resources by thesecond station, generating a primary synchronization signal and asecondary synchronization signal based on the selected cell ID, andsending the primary and secondary synchronization signals in designatedsubframes by the second station.